|Publication number||US8046539 B2|
|Application number||US 12/479,541|
|Publication date||Oct 25, 2011|
|Priority date||Aug 28, 2002|
|Also published as||US7546422, US20040044850, US20090240894|
|Publication number||12479541, 479541, US 8046539 B2, US 8046539B2, US-B2-8046539, US8046539 B2, US8046539B2|
|Inventors||Robert T. George, Mathew A. Lambert, Tony S. Rand, Robert G. Blankenship, Kenneth C. Creta|
|Original Assignee||Intel Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (15), Non-Patent Citations (1), Referenced by (5), Classifications (9), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a continuation of patent application Ser. No. 10/231,414, filed on Aug. 28, 2002, which will issue on Jun. 9, 2009 as U.S. Pat. No. 7,546,422.
The present invention relates to cache memory systems and more particularly to a hierarchical caching protocol suitable for use with distributed caches (e.g., in Very Large-Scale Integration (VLSI) devices), and may be utilized within a caching input/output (I/O) hub.
As is known in the art, the system cache in a computer system serves to enhance the system performance of modern computers. For example, a cache can maintain data between a processor and relatively slower system memory by holding recently accessed memory locations in case they are needed again. The presence of cache allows the processor to continuously perform operations utilizing the data in the faster-accessing cache.
Architecturally, system cache is designed as a “monolithic” unit. In order to give a processor core simultaneous read and write access from multiple pipelines, multiple ports can be added to the monolithic cache device for external I/O devices. However, there are several detrimental architectural and implementation impacts of using a monolithic cache device with several read/write ports (for example, in a dual-ported monolithic cache). Current monolithic cache devices are not optimized for multiple ports and not the most efficient implementation available.
Computer systems are designed to accommodate a single or multiple central processing units (CPUs), coupled via a common system bus or switch to a memory and a number of external input/output devices. The purpose of providing multiple central processing units is to increase the performance of operations by sharing tasks between the processors. Such an arrangement allows the computer to simultaneously support a number of different applications while supporting I/O components that are, for example, communicating over a network and displaying images on attached display devices. Multi-processor computer systems are typically utilized for enterprise and network server systems.
To enhance performance, all of the devices coupled to the bus must communicate efficiently. Idle cycles on the system bus represent time periods in which an application is not being supported, and therefore represent reduced performance.
A number of situations arise in multi-processor computer system designs in which the bus, although not idle, is not being used efficiently by the processors coupled to the bus. Some of these situations arise due to the differing nature of the devices that are coupled to the bus. For example, processors typically include cache logic for temporary storage of data from the memory. A coherency protocol is implemented to ensure that each central processor unit only retrieves the most up to date version of data from the cache. MESI (Modified-Exclusive-Shared-Invalid) coherency protocol data can be added to cached data in order to arbitrate and synchronize multiple copies of the same data within various caches. Therefore, processors are commonly referred to as “cacheable” devices.
However, I/O components, such as those coupled to a Peripheral Component Interconnect (PCI) (“PCI Local Bus Specification”, version 2.1, Jun. 1, 1995, from the PCI Special Interest Group (PCI-SIG)) bus, are generally non-cacheable devices. That is, they typically do not implement the same cache coherency protocol that is used by the processors. Typically, I/O components retrieve data from memory, or a cacheable device, via a Direct Memory Access (DMA) operation. Accordingly, measures must be taken to ensure that I/O components only retrieve valid data for their operations. An I/O device may be provided as a connection point between various I/O bridge components, to which I/O components are attached, and ultimately, to the processor.
An input/output (I/O) device may be utilized as a caching I/O device. That is, the I/O device includes a single, monolithic caching resource for data. Therefore, because an I/O device is typically coupled to several client ports, a monolithic I/O cache device will suffer the same detrimental architectural and performance impacts as previously discussed. Current I/O cache device designs are not efficient implementations for high performance systems.
Implementing multiple cache systems for processors and I/O devices requires cache coherency amongst the caches. Cache coherency is the synchronization of data in a plurality of caches such that reading a memory location via any cache will return the most recent data written to that location via any other cache. Current solutions for synchronizing multiple cache systems include utilizing the MESI coherency protocol and having each cache broadcast its request to every other cache in the system and then wait for a response from those devices. This approach has the inherent problem of being non-scalable. As additional cache devices are added as needed to a multiple cache system, latency throughout the system is increased dramatically, thereby decreasing overall system performance. Multiple cache systems become impractical to implement, and therefore, a need exists for a scalable method of improving the efficiency and latency performance of distributed caches. Furthermore, a need exists for an improved coherency protocol to improve the performance of synchronization of increased data bandwidth inherent in a distributed cache system.
In view of the above, there is a need for a method and apparatus for synchronizing distributed caches in VLSI device, namely, high performance I/O systems.
The cache-coherent CPU device 100 includes a coherency engine 105 and one or more read and write caches 110 and 115. In this embodiment of the cache-coherent CPU device 100, coherency engine 105 contains a reference table, indexing the data within distributed caches 110 and 115. The coherency engine 105 may utilize, for example, the Modified-Exclusive-Shared-Invalid (MESI) coherency protocol, labeling the data with line state MESI tags: ‘M’-state (modified), ‘E’-state (exclusive), ‘S’-state (shared), or ‘I’-state (invalid). In addition, coherency engine 105 may also utilize other coherency protocols, including but not limited to, an improved MESI protocol with intermediate coherency states described further in this application. Each new request from the cache of any of the processor port components 140 and 145 is checked against the table of coherency engine 105. If the request does not interfere with any data found within any of the other caches, the transaction is processed. Utilizing MESI tags enables coherency engine 105 to quickly arbitrate between caches reading from and writing to the same data, meanwhile, keeping all data synchronized and tracked between all caches.
Rather than employing a single monolithic cache, cache-coherent processor device 100 physically partitions the caching resources into smaller, more implementable portions, each connected to coherency engine 105. Caches 110 and 115 are distributed across all ports on the device, such that each cache is associated with a port component. According to an embodiment of the present invention, cache 110 is physically located on the device nearby port component 140 being serviced. Similarly, cache 115 is located proximately to port component 145, thereby reducing the latency of transaction data requests. This approach minimizes the latency for “cache hits” and performance is increased. A cache hit is a request to read from memory that may be satisfied from the cache without using main (or another) memory. Reducing transaction latency is a major benefit of a distributed cache mechanism. This allows the cache coherent device 100 to supply coherent responses to port component devices 140 and 145 without traversing the distance across these devices. However, device 100 must be kept cache coherent with respect to the other coherent devices in the system. This results from host CPU 120 and other coherent port components sending remote coherency “snoops” (i.e. a monitoring signal) to determine if the port component maintains MESI state information for a particular coherent line. Since the coherency engine 105 tracks the MESI state of all coherent lines maintained within the device 100, coherency engine 105 can quickly respond to remote snoop requests. Rather than having the remote snoop request traverse the entire system diameter of each port component to query the local caches directly, the remote snoop is serviced quickly, thereby reducing latency.
Distributing caches, according to this embodiment of the present invention, also provides improvements in scalability design. Using a monolithic cache, an increase in the number of ports would make the CPU device geometrically more complex in design (e.g., a four-port CPU device would be sixteen times more complex using a monolithic cache compared to a one-port CPU device). With this embodiment of the present invention, utilizing a coherency engine, the addition of another port is easier to design into the CPU device by adding the appropriate connections to the coherency engine and an additional cache for the new port. Therefore, distributed caches that implement a coherency directory are inherently more scalable.
Furthermore, the distributed cache architecture improves aggregate bandwidth with each port component 140 and 145 capable of utilizing the full transaction bandwidth for each of the read/write caches 110 and 115.
According to an embodiment of the present invention, a first read cache 215 may be provided and associated with the first I/O interface 235, and a second read cache 220 may be provided and associated with the second I/O interface 240. That is, for each I/O interface 235, 240, there is a read cache 215, 220 dedicated thereto. This approach minimizes the latency for “cache hits” and performance is increased. This arrangement is particularly useful for data that is prefetched by I/O components 245 and 250. Any suitable configuration of the number of I/O interfaces, I/O components, and read caches may be utilized. The read caches 215 and 220 provides the I/O component 245 and 250 a low latency copy of the “most likely to use” data. The distributed cache architecture also improves aggregate bandwidth with each port component 245 and 250 capable of utilizing the full transaction bandwidth for each read/write cache 210, 215 and 220.
In an embodiment of the present invention, a first read cache 215 may be provided and associated with the first I/O interface 235, and a second read cache 220 may be provided and associated with the second I/O interface 240. According to an embodiment of the present invention, the read caches 215 and 220 maintain cache lines in the shared and invalid states. Although the read caches 215 and 220 are shown as being on a single component (i.e., on the cache-coherent I/O device 200), the read caches may be distributed (and separated) on one or more different components. The write cache 210 manages coherent memory writes initiated by the I/O components 245 and 250. The write cache 210 is also responsible for snooping, or monitoring, through coherent host interface 230 to the front-side bus 225 (or an Intel® Scalability Port) before committing to the memory write. According to an embodiment of the present invention, the write cache 210 follows the Modified-Exclusive-Shared-Invalid (MESI) caching protocol. However, any suitable caching protocol may be utilized.
A coherency engine 205 is provided in the cache-coherent I/O device 200. Coherency engine 205 provides tracking of all of the cache lines in the write cache 210 and the read caches 215 and 220. The coherency engine 205 tracks the line state, as well as a pointer to the structure that stores the tracking data. Tracking the state enables a faster response to requests. Tracking the structures storing the line enables the protocol that is required between the structures.
A local request buffer may be provided with the cache-coherent I/O device 200 to store all inbound coherent and non-coherent transactions. A remote request buffer may also be provided with cache-coherent I/O device 200 to check for conflicts between snoop or invalidate cycles from the coherent host and the inbound requests from cache-coherent I/O device 200 itself.
The cache-coherent I/O device 200 may have a plurality of caching resources, each including a write cache 210, a coherency engine 205, one or more read cache 215 and 220, a local request buffer, and a remote request buffer. Moreover, the cache-coherent I/O device 200 may include a coherency engine 205 interconnecting the read caches 215 and 220 and the write cache 210. The coherency engine 205 is utilized to synchronize the caches within the caching resources.
Effective transaction bandwidth in I/O devices is improved in at least two ways by utilizing a cache-coherent I/O device 200. Cache-coherent I/O device 200 may aggressively prefetch data. If cache-coherent device 200 speculatively requests ownership of data subsequently requested or modified by the processor system, caches 210, 215 and 220 may be “snooped” (i.e. monitored) by the processor, which, in turn, will return the data with the correct coherency state preserved. As a result, cache-coherent device 200 can selectively purge contended coherent data, rather than deleting all prefetched data in a non-coherent system where data is modified in one of the prefetch buffers. Therefore, the cache hit rate is increased, thereby increasing performance.
Cache-coherent I/O device 200 also enables pipelining coherent ownership requests for a series of inbound write transactions destined for coherent memory. This is possible because cache-coherent I/O device 200 provides an internal cache which is maintained coherent with respect to system memory. The write transactions can be issued without blocking the ownership requests as they return. Existing I/O devices must block each inbound write transaction, waiting for the system memory controller to complete the transaction before subsequent write transactions may be issued. Pipelining I/O writes significantly improves the aggregate bandwidth of inbound write transactions to coherent memory space.
As seen from the above, the distributed caches serve to enhance overall cache system performance. The distributed caches system enhances the architecture and implementation of a cache system with multiple ports. Specifically within I/O cache systems, distributed caches conserve the internal buffer resources in I/O devices, thereby improving device size, while improving the latency and bandwidth of I/O devices to memory.
In decision block 410, a determination is made whether the check results in a “cache hit” or “cache miss.” If the cache-coherent device does not have exclusive ‘E’ or modified ‘M’ ownership of the cache line, the check results in a cache miss. Control then passes to block 415, where the coherency engine will forward a “request for ownership” to an external coherency device (e.g. memory) requesting exclusive ‘E’ ownership of the target cache line. When exclusive ownership is granted to the cache-coherent device, the coherency engine marks the line as ‘M’. At this point, in decision block 420, the coherency engine may either forward the write transaction data to the front-side bus to write data in coherent memory space in block 425, or maintain the data locally in the distributed caches in modified ‘M’-state in block 430. If the coherency engine always forwards the write data to the front-side bus upon receiving exclusive ‘E’ ownership of the line, then the cache-coherent device operates as a “write-through” cache, in block 425. If the coherency engine maintains the data locally in the distributed caches in modified ‘M’-state, then the cache-coherent device operates as a “write-back” cache, in block 430. In each instance, either forwarding the write transaction data to the front-side bus to write data in coherent memory space in block 425, or maintaining the data locally in the distributed caches in modified ‘M’-state in block 430, control then passes to block 435, where the pipelining capability within distributed caches is utilized.
In block 435, the pipelining capability of global system coherency can be utilized to streamline a series of inbound write transactions, thereby improving the aggregate bandwidth of inbound writes to memory. Since global system coherency will be maintained if the write transaction data is promoted to modified ‘M’-state in the same order it was received from port component 140 or 145 (or similarly from I/O component 245 or 250), the processing of a stream of multiple write requests may be pipelined. In this mode, the coherency engine will forward a request for ownership to an external coherency device requesting exclusive ‘E’ ownership of the target cache line as each write request is received from port component 140 or 145 (or similarly from I/O component 245 or 250). When exclusive ownership is granted to the cache-coherent device, the coherency engine marks the line as modified ‘M’ as soon as all the preceding writes have also been marked as modified ‘M’. As a result, a series of inbound writes from port 140 or 145 (or similarly from I/O component 245 or 250) will result in a corresponding series of ownership requests, with the stream of writes being promoted to modified ‘M’-state in the proper order for global system coherency.
If a determination is made that the check results in a “cache hit” in decision block 410, control then passes to decision block 440. If the cache-coherent device already has exclusive ‘E’ or modified ‘M’ ownership of the cache line in one of the other distributed caches, the check results in a cache hit. At this point, in decision block 440, the coherency engine will manage the coherency conflict either as a write-through cache, passing control to block 445, or, as a write-back cache, passing control to block 455. If the coherency engine always blocks the new write transaction until the senior write data can be forwarded to the front-side bus upon receiving a subsequent write to the same line, then the cache-coherent device operates as a write-through cache. If the coherency engine always merges the data from both writes locally in the distributed caches in modified ‘M’-state, then the cache-coherent device operates as a write-back cache. As a write-through cache, the new write transaction is blocked in block 445, until the older (“senior”) write transaction data can be forwarded to the front-side bus to write data in coherent memory space in block 450. After the senior write transactions have been forwarded, other write transactions can then be forwarded to the front-side bus to write data in coherent memory space in block 425. Control then passes to block 435, where the pipelining capability of distributed caches is utilized. As a write-back cache in block 455, the data from both writes is merged locally in the distributed caches in modified ‘M’-state, and held internally in modified ‘M’-state in block 430. Again, control passes to block 435, where multiple inbound write transactions may be pipelined, as described above.
In state 510, a data line is in ‘I’-state, where both reads and writes typically begin. However, the coherency engine does not have state information for transactions in this state. If the data line is not already present in the cache when a read transaction is issued, the data line is allocated to the cache. The data can then be retrieved from a processor or bus read. After the read transaction is completed, the coherency engine recognizes the data line as ‘S’-state in 515. Once the line is purged or written to the bus, the line is invalid and returned to ‘I’-state in 510. However, when in ‘S’-state, if a snoop detects a line conflict, the coherency engine changes the data line to shared-conflict state as ‘SC’-state in 550 to invalidate the line.
When a write transaction is issued, the line starts as ‘I’-state in 510. The line is requested as exclusive, and then is immediately changed ‘E-prime’ state in 520 awaiting acknowledgement from the coherency engine to promote the line to exclusive ‘E’-state in 525. A request for ownership is issued from the internal ordering queues, but the data line remains in ‘E-prime’ state in 520 until a positive completion for exclusive ownership is received from the coherency engine. If a snoop detects a line in the cache as ‘S’-state, the data line must be invalidated and moves to ‘SC’-state in 550. The line is invalidated and returned to ‘I’-state in 510. Or, if a line conflict is detected in the request for exclusive ownership, the line transitions to ‘SC’ state in 555 and waits for the conflict to be resolved. After the inbound request for ownership is granted, a snoop responds by moving the line to exclusive ‘E’-state in 525.
In ‘E-prime’ state in 520, when the request for ownership is completed, the line moves to ‘E’-state in 525. If a conflict is detected for exclusive access to the line, the line changes to conflict ‘C’-state in 530. A line moves to ‘C’-state as a result of either when a snoop detects a line conflict for exclusive ‘E’-state ownership or when the ordering requirements for a line write have not been met. When the conflict is resolved, the line returns to ‘E-prime’ in 520 awaiting acknowledgment to exclusive access to the line in ‘E’-state in 525.
From ‘E’-state, the line can be modified by writing either a full line writes or a sub-line writes in the form of partial writes to the line. A full line write command completes a processor write and changes the line state to ‘M’-state in 535. This occurs as a result of meeting the ordering requirements for a full line write. The line write to memory is performed and the write transaction is completed. If the data line needs to be written to again, a purge of the cached copy or write to the bus occurs, invalidating the line, and returning the line to ‘I’-state in 510 at the end of the write transaction cycle shown. A sub-line write command includes states 540 and 545, ‘M-prime’ state and modified-exclusive ‘ME-prime’ state, respectively. When the ordering requirements for a sub-line write are met, the line is promoted to ‘M-prime’ state in 540. In this state, the line resides in the write cache, available for subsequent partial writes to merge into it. Two mechanisms can move the data line from ‘M-prime’ state in 540 to ‘M’-state in 535, where the data line is written to memory. Either, an external snoop detects a line conflict in the write cache or the least recently used (LRU) command is issued to purge the overloaded cache line.
When a subsequent sub-line write makes a request for ownership, the data line in ‘M-prime’ state in 540 is moved to ‘ME-prime’ state in 545. In this pseudo-exclusive state, similar to the ‘E-prime’ state, the data line may move to ‘C’-state in 530 as a result of either a snoop detecting a line conflict for exclusive ownership for the partial write or ordering requirements for a partial write not being met. If the exclusive ownership for the partial write is granted and the ordering requirements are met for a partial line, the data line changes back to ‘M-prime’ state in 540 to await further sub-line write commands. However, the data line in ‘ME-prime’ state in 545 may need to be written to memory. If the ordering requirements for a full line write are met, the partial lines are merged for a full line write. The line write to memory is performed and the write transaction is completed as the data line is promoted from ‘ME-prime’ state in 545 to ‘M-state’ in 535. Once the line is written to memory, the coherency engine no longer holds the state for the line, and is considered invalid ‘I’-state in 510.
The intermediate MESI states, ‘E-prime’ in state 520, ‘C’-state (conflict) in state 530, ‘M-prime’ in state 540, and ‘ME-prime’ (modified/exclusive) in state 545 are discussed in further detail in
If no ownership conflict exists for the data line, control passes to block 625, where a full line write commands is issued. A processor write occurs and the line is converted to modified ‘M’-state.
If no conflicts result from the request for ‘ME-prime’ ownership, the line is placed in ‘ME-prime’ state and the partial write occurs in block 715. Control then passes to decision block 730. If the system issues a full line write command, the partial line writes are merged into a full line write. The line is merged into a full line write to the processor and the line state converted to modified ‘M-state’ in block 735. If there is no full line write command, control passes to block 740 so the line returns to ‘M-prime’ state awaiting additional partial writes to the line. However, if a snoop detects a line conflict in the write cache or a least recently used (LRU) command is issued, the line is written as a full line write. The LRU command eliminates the least used cached entry if the number of partials writes outnumber the write capacity of the full line write. The line converts from ‘M-prime’ state to modified ‘M’-state.
As discussed above, the cache-coherent I/O device 200 may include input/output interfaces 235 and 240 that are coupled to an intermediary device, such as a hub-link or input/output bridge, like a PCI-X bridge 860 or an InfiniBand bridge 870. The I/O components or devices 245 and 250 then connect to the intermediary devices 860 and 870. Cache-coherent I/O device 200 may also include an I/O interface that connects to a legacy input/output bridge 850 to handle connections with legacy I/O components or devices.
The cache-coherent I/O device 200 is adapted to connect to a coherent host, such as a Scalability Port 840, which is a cache-coherent interface optimized for scalable multi-node systems that maintain coherency between all processors and their caches. The Scalability Port 840 in turn may connect to at least one Scalability Node Controller 820, which controls the interface between the processors 810, the main memory 830 (e.g., DRAM), and the Scalability Port 840.
Accordingly, the cache-coherent I/O device 200 utilized in the I/O system architecture 800 of the present invention provides for optimized I/O applications in, for example, a chipset for a multi-processor computer system. The cache-coherent I/O device 200 minimizes latency and provides quick responses to requests. Implementation scalability and performance are therefore optimized with the cache-coherent I/O device 200 of the present invention.
While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
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|U.S. Classification||711/141, 711/E12.026, 711/144, 711/146|
|Cooperative Classification||G06F12/082, G06F12/0835|
|European Classification||G06F12/08B4P4P, G06F12/08B4P2A|